1. Field of the Invention
This invention relates to the field of protective coatings for use on nickel and cobalt base alloys, particularly at high temperatures, to reduce oxidation/corrosion.
2. Description of the Prior Art
Nickel and cobalt base superalloys are widely used under conditions of high temperature where oxidation/corrosion are serious problems. Such alloys find particular use in the field of gas turbine engines, where increased efficiency can be obtained by operation at higher temperatures. Under such increased temperatures oxidation/corrosion becomes a greater problem and for this reason current gas turbine engine practice is to use protective coatings on a majority of nickel and cobalt alloy parts which are used at elevated temperatures. The term "oxidation/corrosion" is meant to refer to high temperature interactions between the superalloy or coated superalloy and the environment. The major active element is oxygen, however corrosive effects can result from other elements such as sodium, sulfur, vanadium and other alkali metals or alkali metal compounds. The most successful known type of coatings are those which rely on the formation of a continuous layer comprised predominately of aluminum oxide (Al.sub.2 O.sub.3) on the surface of the coating which acts as a diffusion barrier to minimize further reactions. Alumina has been found to be the most effective protective material with regard to oxygen and is also beneficial with regard to most of the other reactive environmental elements. The function of protective coatings is to form a barrier which minimizes the reaction of the environment with the superalloy base material. This function is performed by the formation of an alumina layer on the coating surface. A major problem encountered with such coatings is that the coefficient of thermal expansion of the alumina layer differs from the coefficient of expansion of the base material and the coating material which are generally similar. During thermal cycling stresses develop between the alumina layer and the coating material. The alumina layer, which is relatively brittle, tends to crack and spall off thus exposing a fresh surface to the deleterious atmosphere. This repeated formation and spallation of the oxide layer causes the reduction of the coating material in aluminum content. When the aluminum level of the coating material drops below a certain point the coating becomes ineffective as an alumina former and the protective benefits of the coating material are lost.
It has been found in the past that the addition of yttrium to the coating material improves the adherence of the alumina layer to the surface of the coating material. Alumina forming coating materials containing yttrium are described in U.S. Pat. Nos. 3,528,861, 3,542,530, 3,649,225 and 3,676,085 all of which are assigned to the assignee of the present invention.
U.S. Pat. No. 3,102,044, assigned to the present assignee, describes a method of applying coatings and includes a description of an aluminum-10% silicon coating system. Both the coating application technique and the coating composition are fundamentally from the invention which will be described herein. U.S. Pat. No. 3,741,791, also assigned to the present assignee describes a coating which contains chromium, aluminum, yttrium and 10- 16% silicon in a cobalt/nickel base. This relatively large silicon addition is made in order to depress the melting point thus aiding in the diffusion of the coating into the substrate. No particular enhancement of oxide adherence is contemplated by the patent.